Let $ x $ be the common difference. Then $ a = c - 2x $, $ b = c - x $, $ d = c + x $, and $ e = c + 2x $, so \[a + b + c + d + e = (c - 2x) + (c - x) + c + (c + x) + (c + 2x) = 5c.\]But this sum is also 30, so $ 5c = 30 $, which means $ c = 6 $. Therefore, the answer is $ \boxed{\text{(C)}} $. To see that the values of the other terms cannot be found, note that the sequences 4, 5, 6, 7, 8 and 10,8,6,4,2 both satisfy the given conditions.